A lot of leadless electronic devices are surface-mounted on a circuit board. On the other hand, in regard to some electronic devices, such as a connector and a variable resistor, their leads are inserted and soldered into through holes as electronic insert mounting devices. In this description, the through hole means a hole such as a plated through hole, a wall surface of which is covered by conductive film.
FIG. 1A is a plan view showing a structure of a portion of the conventional circuit board where electronic insert mounting devices are mounted, and FIG. 1B is a cross-sectional view taken along line A-A in FIG. 1A.
The circuit board that is used to mount electronic devices is usually manufactured by undergoing the following steps. First, a prepreg in which resin such as epoxy resin and polyimide resin is impregnated into a glass fabric base material and the resin is partially-hardened, or a prepreg in which phenol resin is impregnated into a paper base material and the resin is partially-hardened, and a copper-clad laminate in which copper foil is affixed on the prepreg by the pressure and heat treatment, are prepared. Then, the copper foil that is affixed on the copper-clad laminate is patterned by the photo etching method or the like, and a predetermined number of wiring substrates having copper foil patterns to be internal layer wires 3 are fabricated. Successively, in order to improve adhesion to the prepreg, the roughing process (blackening process) is applied to the copper foil surface of the wiring substrate. After that, these wiring substrates are laminated through the prepreg so that the wiring circuits are the outermost layers and are integrated by applying pressure and heat, and then resin laminate 2 having internal layer wires 3 inside is manufactured.
Then, holes which will become through holes 4 are opened in resin laminate 2 by the drilling process. Successively, in order to improve the connection between internal wires 3 and through holes 4, resin at internal wires 3 is cleaned (desmeared). After that, the activation process, the electroless plating process, and the electrolytic plating process are performed to form through holes 4, in each of which a conductive film is formed on a wall surface of the hole. Then, after through holes 4 are protected by the hole plugging process or the tenting process, the outermost copper layer is patterned to form external wires 5. At this time, lands 6 are formed around the openings of through holes 4 in both surfaces of the board. Through holes 4, external wires 5, and lands 6 may be formed by the pattern plating method.
Finally, although not shown, solder resists are formed in areas except the solder portions in both surfaces of the board, and then the manufacturing process of multi-layer circuit board 1 is completed.
The above is the manufacturing process of multi-layer circuit board 1 having internal wires 3. Alternatively, a double-sided circuit board may be formed by using a double-sided copper-clad plate as a starting material and by similarly performing the through hole forming step and subsequent steps.
In FIGS. 1A and 1B, a dashed-dotted line indicated by symbol ◯ shows the centerline of a case of an electronic device at the time when the electronic device is mounted. In the example shown in FIGS. 1A and 1B, the electronic device has an elongated plane shape with leads arranged in a line, and centerline ◯ is the centerline in the longitudinal direction of the electronic device. In the conventional circuit board, all the shapes and sizes of through holes 4 are equal without making a distinction between center through hole 4a, into which a lead closest to the center of the case of the electronic device is inserted, and outermost end through hole 4b, into which a lead at the outermost end of the case of the electronic device is inserted.
In the process of soldering the electronic device on multi-layer circuit board 1, which is manufactured like this, generally, after the reflow process of mounting surface-mounted devices, such as chip devices and QFP, is performed, the flow process of mounting electronic insert mounting devices is performed.
As solder materials used to solder electronic devices, tin-lead solder, in particular, tin-lead eutectic solder which is close to an eutectic composition and which has the mass ratio of Sn and Pb, like Sn:PB=60 to 63%:40 to 37%, has been used for a long time. Since tin-lead eutectic solder is a material having high ductility, during the soldering process or the like, the tin-lead eutectic solder can absorb the stress generated by differences of thermal expansions and thermal shrinkages between multi-layer circuit board 1 and the case of the electronic device or the like, and the solder can reduce the stress applied to multi-layer circuit board 1 and the electronic devices.
However, in recent years, due to increasing awareness of environmental issues, environment pollution caused by lead becomes problematic, and a shift to lead-free solder is rapidly promoted. This lead-free solder mainly includes tin and additionally includes silver, copper, zinc, bismuth, indium, antimony, nickel, germanium, and so on, and has characteristics that it is stronger in metal tensile strength and creep strength and is smaller in ductibility than conventional eutectic solder [typically, Sn is 63% (mass ratio) and the rest is Pb]. Also, the melting temperature of lead-free solder is relatively high, 190 to 230° C., whereas that of tin-lead eutectic solder is 183° C. Therefore, when lead-free solder is used, the stress generated by difference in thermal expansions and thermal shrinkages between the multi-layer circuit board and the case of the electronic device during the soldering process or the like is increased and the stress reduction effect by solder is decreased, and therefore the stress applied to the circuit board is increased. For that reason, the occurrence rate of the phenomenon in which the circuit board is broken, in particular, at the through hole portion at the outermost end of the electronic device, is increased. In other words, although such a phenomenon may occur when conventional tin-lead eutectic solder is used, the occurrence of this phenomenon is increased after the shift to lead-free solder.
More specific explanations are given of such a phenomenon, in which the circuit board is broken, with reference to FIGS. 2, 3A to 3C. FIG. 2 is a cross-sectional view showing a state in which an electronic device is soldered on conventional multi-layer circuit board 1 shown in FIGS. 1A and 1B by using lead-free solder. Also, FIGS. 3A to 3C are enlarged cross-sectional views showing the portion of outermost end through hole 4b in FIG. 2. These cross-sectional views are drawings based on cross-sectional photographs when an electronic device having a polyamide case and provided with a connector having 8-pins arranged in one-line or leads is soldered with lead-free solder (Sn-3.0Ag-0.5Cu) on a circuit board, which has FR-4 as a base material.
As shown in FIG. 2, each of leads 8 extending from case 7 of the electronic device is inserted into each of through holes 4 and is electrically and mechanically connected to a conductive film on the internal wall surface of each of through holes 4 and each of lands 6 with solder fillet 9. At this time, a lead of leads 8, which is inserted into center through hole 4a, is soldered so that its center almost coincides with the center of center through hole 4a and it extends almost vertically relative to multi-layer circuit board 1. On the other hand, a lead of leads 8, which is inserted into outermost end through hole 4b, is soldered so that its tip is positioned away from the center of outermost end through hole 4b in a direction (outside direction) opposite to the side of centerline ◯ of case 7 of the electronic device at the time when it is mounted, and the lead is tilted in a direction toward centerline ◯ of case 7 of the electronic device with progression toward the foot side (upper side in the drawing) of the corresponding lead of leads 8.
The reason that the electronic device is mounted like this is that case 7 of the electronic device and multi-layer circuit board 1 are different in materials and thus are different in thermal expansion coefficients, in particular, in the example shown in the drawings, the thermal expansion coefficient of case 7 of the electronic device is larger than that of multi-layer circuit board 1. Before the soldering process is performed, in outermost end through hole 4b, the center thereof almost coincides with the center of the corresponding lead of leads 8. However, during the soldering process, case 7 of the electronic device is thermally expanded by a larger amount than multi-layer circuit board 1, and therefore the relative position of case 7 of the electronic device and multi-layer circuit board 1 is shifted. The father is the position from centerline ◯ of case 7 of the electronic device, the larger is the shift. Therefore, the outermost end lead of leads 8 is soldered on the position that is considerably shifted in a direction opposite to a direction from the center of outermost end through hole 4b to the center of case 7 at the time when the electronic device is mounted. Then, after the soldering process, as the temperature lowers, case 7 of the electronic device is thermally shrunk by a larger amount than multi-layer circuit board 1, and thus case 7 pulls the outermost end lead of leads 8 toward the side of the center of case 7 so that the outermost end lead of leads 8 is tilted in the direction toward centerline ◯ of case 7 with progression toward the upper side in the drawings.
As shown in FIG. 3A, the outermost end lead of leads 8 is soldered at the position that is shifted in the direction opposite to the direction from the center of outermost end through hole 4b toward the center of case 7 at the time when the electronic device is mounted, and therefore, the amount of the solder in an area (A-portion: shaded area in the drawing) in the direction opposite to the center direction of the case of the electronic device, between through holes 4 and leads 8, is reduced. Then, due to temperature drop after the soldering process, leads 8 are tilted to the center of case 7 with progression toward the upper side in the drawings. At this time, because the amount of the solder in A-portion is small as abovementioned and lead-free solder has lower stress reduction capacity than the tin-lead eutectic solder, the effect that the lead-free solder absorbs and releases the stress generated by tilting of leads 8 is extremely small. Therefore, great stress is applied to corner portion B and internal wall surface C in the portion in the direction opposite to the center of the case at the time when the electronic device is mounted. Accordingly, as shown in FIG. 3B, corner crack 11 is apt to occur at corner portion B of outermost end through hole 4b, and, as shown in FIG. 3C, through hole separation 12, in which the conductive film covered on the internal wall surface of outermost end through hole 4b peels, is apt to occur. In such a situation, faulty electrical continuity of the electronic device will occur.